1. Field of the Invention
This invention relates to a metal gasket used to seal a clearance between opposed surfaces of parts of a multicylinder engine.
2. Description of the Prior Art
A metal gasket formed out of a metallic material has heretofore been used to seal a clearance between opposed surfaces of a cylinder head and a cylinder block of an engine. In a recent engine, the attaining of a higher output and the reduction of the weight have been demanded, and a cylinder head and a cylinder block have a tendency to be formed out of an aluminum material of a small specific gravity instead of steel and a cast conventionally used, for meeting the demand. The aluminum engine structural members have a small weight but a lower rigidity, so that the relative displacement of a cylinder head and a cylinder block during an operation of the engine tends to increase.
Various types of metal gaskets for sealing a clearance between opposed surfaces of the two structural members have been provided. A metal gasket is provided with beads formed along the portions thereof opposed to the circumferences of cylinder bores (combustion chambers) and passages for water and oil, and these beads form annular elastic contact portions with respect to the opposed surfaces when the cylinder head and cylinder block are tightened and fixed by bolts so as to seal a clearance between the opposed surfaces of the two parts.
During the combustion cycles of an engine, the height of a clearance between the cylinder head and cylinder block increases and decreases repeatedly, and stress, i.e. mechanical stress and thermal stress are exerted repeatedly on a metal cylinder head gasket. The load fluctuation stress of an engine shows a high value in the portions of the cylinder block which have the lowest rigidity. Consequently, permanent set in fatigue and cracks occur in the beads formed on a bead base plate, and the sealing performance of the metal gasket is deteriorated.
A conventional metal cylinder head gasket is disclosed in Japanese patent Laid-Open No. 61506/1996. This metal cylinder head gasket is formed by laminating in order a bead base plate provided with beads along the circumferences of holes and comprising an elastic metal plate, a regulating plate provided with beads along the circumferences of holes and comprising an elastic metal plate, and a hole-carrying grommet plate, the inner circumferences of the holes of the regulating plate being held and fixed by the grommet plate to form parallel-arranged holes, the portions of the beads on the bead base plate which are between adjacent holes comprising full beads, the portions of the beads on the same plate which extend along the remaining parts of the circumferences of the holes comprising half beads, the beads on the regulating plate comprising full beads extending along the whole of the circumferences of the holes, the bead base plate being laminated on the regulating plate so that the beads on the former plate are superposed on those on the latter plate in the regions thereof which are between adjacent holes.
In a metal gasket, it is preferable to concentrate a surface pressure (face-to-face pressure) to a certain extent on the regions thereof which are around combustion chamber holes, and thereby secure an average surface pressure condition in which a necessary surface pressure is secured around the water holes and oil holes, for the purpose of improving the surface pressure balance and sealability of the gasket.
However, in the conventional metal gasket, the surface pressure is concentrated excessively on the grommet portions, and a necessary surface pressure cannot be secured around the water holes and oil holes. Consequently, securing an average surface pressure becomes difficult, and an excellent sealed condition cannot be maintained. When the surface pressure concentration occurs in the bead-formed portions of a metal gasket, a surface pressure balance is deteriorated, and the sealability thereof lowers.
In a conventional metal gasket, the projecting surfaces of the beads directly contact a cylinder head or a cylinder block when the gasket is inserted between the cylinder head and cylinder block and tightened therebetween, and the regions alone in which the projecting surfaces of the beads contact the cylinder head or cylinder block generate a high surface pressure linearly. Therefore, impressions and breakage occur in the portions of the cylinder block or cylinder head which the projecting surfaces of the beads contact, and, moreover, the sealing performance of the gasket lowers.